As an interest in energy and environment is increased, the demand for secondary cells is sharply increased. Among them, a lithium secondary cell having high energy density and discharge voltage is actively being researched and developed.
Particularly, the lithium secondary cell is largely used as a power source of cellular phones, notebook computers, digital cameras, and electric cars.
Meanwhile, the lithium secondary cell has excellent electric properties but low safety.
More specifically, in the lithium secondary cell, heat and gas are generated when a degradation reaction is continuously induced under abnormal conditions, such as overcharge, overdischarge, exposure to high temperature, short circuits, or the like. Here, as the degradation reaction is further promoted under high temperature and high pressure due to the above conditions, the lithium secondary cell may be ignited or exploded.
These problems may bring about more severe large accidents in a large and medium sized battery pack having plural battery cells.
The large and medium sized battery pack has a plurality of battery cells or unit modules within a frame having a predetermined space thereof. As the plurality of battery cells and unit modules swell, the pressure inside a case may be rapidly increased.
A fuse, a bimetal, and a battery management system (BMS) are provided in the secondary cell module in order to solve the problems, but these constitutions can not secure sufficient safety.
In particular, the BMS senses electric errors (overdischarge, overcharge, and overcurrent) at the normal state to control the overall module, thereby securing safety. However, when the BMS is not operated at the abnormal state, the overall control thereby is difficult, and thus, the plurality of battery cells swell, resulting in high risk of ignition or explosion.
Moreover, in a case of a pouch type battery cell in which the electrolytic solution is injected into all-integrated cells, when each cell is over-discharged, voltage becomes increased and the electrolytic solution within the cell becomes degraded due to overheat, and thus, inflammable gas is generated within the cell and an inner pressure of the pouch is increased, resulting in a swelling phenomenon in which the pouch itself swells.
Further, a separator between a cathode and an anode melts, which causes the cathode and the anode to be short-circuited, resulting in ignition, and thus, safety of the cell can not be secured.
Due to these safety problems of the lithium secondary cell, various thermal safety tests such as, a high-temperature storage test, a thermal shock test, a thermal exposure test, as well as safety tests such as over-discharging and compulsory discharging, are conducted before the completed cell are put on the market. According to these thermal safety tests, the cell is left for several tens of minutes to several tens of hours under various temperature conditions. Here, the cell should not be exploded and ignited, or in extreme cases, the cell should be unsealed to prevent explosion and ignition thereof.
The related arts disclose techniques for solving the above-described problem that an inside pressure of the pouch type secondary cell is increased due to an excess amount of gas generated by overcurrent.
Korean Patent No. 0560158 (“Lithium Secondary Cell”, hereinafter, Document 1) discloses a structure functioning as an anti-explosion safety member in which an adhesive layer is partially formed on an outer surface of a packing of the cell, and thus, the packing is abruptly ruptured to prevent explosion of the cell when an inner pressure of the cell is increased to above a critical value to swell the packing.
According to Document 1, the above-described problem can be solved to some degree because abrupt explosion is prevented. However, since gas is exhausted when the cell swells and then a portion of the packing, on which the adhesive layer is not formed, is finally ruptured, explosion itself still occurs although the intensity of cell explosion is decreased, therefore failing to solve the problems completely.
Therefore, various techniques for preventing explosion or ignition at the time when swelling of the lithium secondary cell occurs are needed.